body {
  margin: 0px;
}
.flex-box {
  display: flex;
}
.border-box {
  box-sizing: border-box;
  border: 1px solid #ff0000;
}

.mybox1 {
  flex-direction: column;
  height: 10rem;
}

.flex1 {
  flex: 1;
}

.mybox2 {
  flex-direction: column;
  height: 40rem;
  justify-content: space-between;
}

.fixed-box {
  position: fixed;
  text-align: center;
  top: 0px;
  left: 0px;
  width: 100%;
  font-size: 1.2rem;
}
